Ink-jet recording method

ABSTRACT

The present invention relates to an ink-jet printing method that is capable of obtaining good printed characters or images which are free of thermal deformation of a low-liquid absorbing printing medium as well as occurrence of mottling or color bleeding. The present invention provides an ink-jet printing method including the step of printing characters or images on a low-liquid absorbing printing medium using an ink-jet printing apparatus equipped with an ink-jet print head that is capable of ejecting water-based inks onto the low-liquid absorbing printing medium, in which the water-based inks each contain a colorant (A), an organic solvent (C) and water; the ink-jet printing apparatus is loaded with the two or more kinds of water-based inks which are different in static surface tension from each other; the printing medium is heated to a temperature of from 30 to 75° C.; and the water-based inks are ejected in the sequential order from the water-based ink having a higher static surface tension.

FIELD OF THE INVENTION

The present invention relates to an ink-jet printing method.

BACKGROUND OF THE INVENTION

In ink-jet printing methods, droplets of ink are directly projected ontoa printing medium from very fine nozzles and allowed to adhere to theprinting medium to form characters or images thereon. The ink-jetprinting methods have become rapidly spread because of variousadvantages such as easiness of full coloration, low cost, capability ofusing a plain paper as the printing medium, non-contact with printedcharacters or images, etc.

On the other hand, it has been required to print characters or imagesnot only on a high-liquid absorbing printing medium such as thoseprinting media conventionally called a plain paper or a copying paper,etc., but also on a printing medium for commercial printing purposessuch as a low-liquid absorbing coated paper such as an offset coatedpaper or a non-liquid absorbing resin film such as a polyvinyl chlorideresin film, a polypropylene resin film and a polyester resin film.

It is known that when characters or images are printed on the low-liquidabsorbing or non-liquid absorbing printing medium by the ink-jetprinting methods, there tend to occur problems such as prolonged dryingtime of ink owing to slow absorption or no absorption of liquidcomponents of the ink, and deterioration in rub fastness of the printedcharacters or images in an initial stage of the printing.

Hitherto, when printing characters or images on the low-liquid absorbingor non-liquid absorbing printing medium, a solvent-based pigment inkusing an organic solvent as a dispersing medium and a UV-curing ink havebeen mainly employed. This is because by using such a phenomenon thatthe organic solvent is penetrated into the low-liquid absorbing ornon-liquid absorbing printing medium and causes swelling of the surfaceof the printing medium to thereby fix the pigment in the printingmedium, or by forming a firm resin coating on the printing medium afterprinting, it is possible to improve anti-bleeding properties and rubfastness of these inks.

However, these conventional inks tend to have such a problem that alarge amount of the organic solvent is diffused into air upon drying theink to cause adverse influence on environments, or a radical initiatoror a monomer used in the UV-curing ink poses a risk concerning safety.For this reason, at the present time, development of a water-based inkhaving a less burden on working environments and natural environmentshas proceeded.

On the other hand, almost all of conventional water-based inks have sucha problem that they are incapable of forming good printed characters orimages on a low-liquid absorbing or non-liquid absorbing printingmedium.

JP 2007-331171A (Patent Literature 1) discloses an image-forming methodin which dots of inks are stabilized in shape to form good characters orimages, which includes the step of first applying a reaction solutioncontaining a component that is reactive with inks onto a printing mediumand then applying the plural kinds of inks onto the printing medium inthe sequential order from the ink having a higher surface tension toform the printed characters or images thereon.

JP 2012-224658A (Patent Literature 2) discloses an ink-jet printingmethod using a water-based ink as a method of forming good characters orimages which are free of mottling (uneven density of ink) or occurrenceof white spots on a non-absorbing printing medium, in which anevaporation rate and a surface tension of ink fall within respectivespecific ranges; the ink contains two or more kinds of organic solvents;the ink has such a solubility parameter that a sum of a hydrogen bondingterm and a polarity term falls within a specific range; and a content ofa surfactant in the ink is less than a critical micelle concentration ofthe surfactant.

SUMMARY OF THE INVENTION

The present invention relates to an ink-jet printing method includingthe step of printing characters or images on a low-liquid absorbingprinting medium using an ink-jet printing apparatus equipped with anink-jet print head that is capable of ejecting water-based inks onto thelow-liquid absorbing printing medium, in which:

the water-based inks each contain a colorant (A), an organic solvent (C)and water;

the ink-jet printing apparatus is loaded with the two or more kinds ofwater-based inks which are different in static surface tension from eachother;

the printing medium is heated to a temperature of from 30 to 75° C.;

and the water-based inks are ejected in the sequential order from thewater-based ink having a higher static surface tension.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view showing a print pattern used for evaluating printedmaterials obtained in respective Examples, etc.

DETAILED DESCRIPTION OF THE INVENTION

In the Patent Literature 1, there is described the technology in whichby applying the inks onto the printing medium in the sequential orderfrom the ink having a higher surface tension, occurrence of such aphenomenon that the preceding ink droplets having been applied onto theprinting medium are pushed aside by the subsequent ink droplets impactedlater thereon can be reduced, so that it is possible to suppressdisturbance of a shape of ink dots. In the Patent Literature 1, althoughthe influence exerted on the printed characters or images by a shape ofthe ink dots and misregistration of the ink droplets impacted wasevaluated, there has been made no concrete study on color bleeding ofthe inks. In addition, in the paragraph [0013] of the Patent Literature1, it is clearly described that the color bleeding is suppressed by aflocculating effect of the inks which is obtained using the reactionsolution, but if no reaction solution is used, considerabledeterioration in quality of the printed characters or images is causedowing to the color bleeding.

Also, the ink-jet printing method described in the Patent Literature 2tends to fail to achieve the task of obtaining good printed charactersor images which are free of mottling or color bleeding on the non-liquidabsorbing printing medium to a fully satisfactory extent.

The present invention relates to an ink-jet printing method in whichgood printed characters or images which are free of mottling or colorbleeding can be formed on a low-liquid absorbing printing medium withoutcausing any thermal deformation of the low-liquid absorbing printingmedium irrespective of a flocculating effect of inks.

The present inventors have found that in an ink-jet printing method inwhich characters or images are printed on a low-liquid absorbingprinting medium, by heating the printing medium to a temperature of from30 to 75° C. and ejecting two or more kinds of water-based inks whichare different in static surface tension from each other in thesequential order from the ink having a higher static surface tension, itis possible to solve the aforementioned conventional problems.

That is, the present invention relates to an ink-jet printing methodincluding the step of printing characters or images on a low-liquidabsorbing printing medium using an ink-jet printing apparatus equippedwith an ink-jet print head that is capable of ejecting water-based inksonto the low-liquid absorbing printing medium, in which:

the water-based inks each contain a colorant (A), an organic solvent (C)and water;

the ink-jet printing apparatus is loaded with the two or more kinds ofwater-based inks which are different in static surface tension from eachother;

the printing medium is heated to a temperature of from 30 to 75° C.; and

the water-based inks are ejected in the sequential order from thewater-based ink having a higher static surface tension.

In accordance with the present invention, there is provided an ink-jetprinting method in which good printed characters or images which arefree of mottling or color bleeding can be formed on a low-liquidabsorbing printing medium without causing any thermal deformation of thelow-liquid absorbing printing medium.

[Ink-Jet Printing Method]

The ink-jet printing method of the present invention is characterized byincluding the step of printing characters or images on a low-liquidabsorbing printing medium (hereinafter also referred to merely as a“printing medium”) using an ink-jet printing apparatus equipped with anink-jet print head that is capable of ejecting water-based inks(hereinafter also referred to merely as “inks”) onto the low-liquidabsorbing printing medium, in which:

the water-based inks each contain a colorant (A), an organic solvent (C)and water;

the ink-jet printing apparatus is loaded with the two or more kinds ofwater-based inks which are different in static surface tension from eachother;

the printing medium is heated to a temperature of from 30 to 75° C.; and

the water-based inks are ejected in the sequential order from thewater-based ink having a higher static surface tension.

In the ink-jet printing method of the present invention, all of thewater-based inks used in the method are ejected in the sequential orderfrom the ink having a higher static surface tension.

Meanwhile, the term “printing” as used in the present specification is aconcept that includes printing or typing operation for printingcharacters or images, and the term “printed material” as used in thepresent specification is a concept that includes printed matters ortyped materials on which characters or images are printed.

In addition, the term “low-liquid absorbing” as used in the presentspecification is intended to include both concepts of “low-liquidabsorbing properties” and “non-liquid absorbing properties” for waterand/or ink, and the low-liquid absorbing printing medium may beevaluated by determining a water absorption of the printing medium usingpure water. More specifically, the low-liquid absorbing printing mediummeans a printing medium having a water absorption of not less than 0g/m² and not more than 10 g/m², preferably not less than 0 g/m² and notmore than 6 g/m², as measured under the condition that a contact timebetween the printing medium and pure water is 100 milliseconds.Incidentally, the aforementioned water absorption of the printing mediummay be measured by the method described in Examples below.

Furthermore, the term “aqueous medium” as used in the presentspecification means such a medium that water has a largest content amongcomponents of the medium contained in the ink. The aqueous medium may beconstituted of water solely, or may be in the form of a mixed solventcontaining water and at least one organic solvent.

The ink-jet printing method of the present invention exhibits such anexcellent effect that good printed characters or images which are freeof mottling or color bleeding can be formed on a low-liquid absorbingprinting medium without causing thermal deformation of the low-liquidabsorbing printing medium. The reason why the aforementionedadvantageous effect can be attained by the present invention isconsidered as follows though it is not clearly determined yet.

That is, as described in the present invention, in the case where thelow-liquid absorbing printing medium is heated in the printing step,droplets of the ink precedingly impacted onto the printing medium aredried and concentrated during the period until the droplets of the otherink having a different color are subsequently impacted thereon. When thewater-based ink is dried and concentrated, water in the water-based inkis evaporated prior to evaporation of the solvent therein to therebycause a considerable change in composition of the ink. The change incomposition of the ink in turn causes gradual reduction of a staticsurface tension of the ink. For this reason, when the next ink that isstill kept in a non-concentrated state is impacted on the printingmedium adjacent to the ink precedingly impacted, droplets of these inksthat are different in static surface tension from each other are broughtinto contact with each other. When causing contact between the dropletsof the inks that are different in static surface tension from eachother, a wetting action of the droplets of the inks takes place alongtheir interface, which tends to result in occurrence of color bleedingbetween the inks. In consequence, if the static surface tension of theink subsequently impacted on the printing medium is adjusted in advanceso as to be lower than that of the ink precedingly impacted thereon, itis possible to reduce a difference in static surface tension between theinks owing to drying and concentration of the inks. Thus, in the presentinvention, it is considered that by reducing the difference in staticsurface tension between the respective color inks during the heating anddrying step, it is possible to suppress occurrence of mottling or colorbleeding of these inks.

In addition, it is considered that by using an organic solventcontaining a glycol ether as the organic solvent (C) and, if required,by further incorporating a silicone-based surfactant as the surfactant(D) into the water-based ink, it is possible to further improveuniformity of printed characters or images owing to spread of ink dotswhen printed on a non-liquid absorbing printing medium by an ink-jetprinting method, and therefore it is possible to obtain good printedcharacters or images which are free of mottling.

<Water-Based Inks>

The water-based inks used in the present invention each contain thecolorant (A), the organic solvent (C) and water.

<Colorant (A)>

As the colorant (A) in the respective water-based inks, there may beused either a pigment or a dye. From the viewpoint of imparting goodweathering resistance and good water resistance to printed materials,the pigment is preferably used.

The pigment used in the present invention may be either an inorganicpigment or an organic pigment. The inorganic or organic pigment may alsobe used in combination with an extender pigment, if required.

Examples of the inorganic pigment include carbon blacks, metal oxides,metal sulfides and metal chlorides. Of these inorganic pigments, inparticular, carbon blacks are preferably used for black inks. The carbonblacks may include furnace blacks, thermal lamp blacks, acetylene blacksand channel blacks. In addition, examples of the inorganic pigment for awhite ink include metal oxides such as titanium oxide, zinc oxide,silica, alumina and magnesium oxide, etc. Among these inorganic pigmentsfor a white ink, preferred is titanium oxide.

Examples of the organic pigment include azo pigments, diazo pigments,phthalocyanine pigments, quinacridone pigments, isoindolinone pigments,dioxazine pigments, perylene pigments, perinone pigments, thioindigopigments, anthraquinone pigments and quinophthalone pigments. The hue ofthe organic pigment used in the present invention is not particularlylimited, and there may be used any chromatic organic pigment having ayellow color, a magenta color, a cyan color, a blue color, a red color,an orange color, a green color, etc. Specific examples of the preferredchromatic organic pigments include one or more pigments selected fromthe group consisting of commercially available products marketed underthe names of C.I. Pigment Yellow, C.I. Pigment Red, C.I. Pigment Orange,C.I. Pigment Violet, C.I. Pigment Blue and C.I. Pigment Green withvarious product numbers.

The aforementioned pigments and dyes may be respectively used alone orin the form of a mixture of any two or more thereof.

The pigment used in the respective water-based inks may be employed inthe form of at least one pigment selected from the group consisting of aself-dispersible pigment, and particles formed by dispersing a pigmentwith the polymer (B).

The polymer (B) may be used as a pigment dispersing polymer (B-1) fordispersing the pigment and a fixing aid polymer (B-2) for improvingfixing properties of the resulting printed characters or images. Thesepolymers (B-1) and (B-2) may be used in combination with each other.

From the viewpoint of obtaining good printed characters or images whichare free of mottling or color bleeding, the pigment is preferablycontained in the respective water-based inks in the form of particles ofa pigment-containing water-insoluble polymer (BX).

[Self-Dispersible Pigment]

The self-dispersible pigment means a pigment onto a surface of which atleast one hydrophilic functional group (including an anionic hydrophilicgroup such as a carboxy group or a cationic hydrophilic group such as aquaternary ammonium group) is bonded either directly or through theother atom group such as an alkanediyl group having 1 to 12 carbon atomsto thereby render the pigment dispersible in an aqueous medium withoutusing a surfactant or a resin.

Specific examples of commercially available products of theself-dispersible pigment include “CAB-O-JET” series products availablefrom Cabot Japan K.K. The self-dispersible pigment is preferably used inthe form of a pigment water dispersion prepared by dispersing thepigment in water.

[Particles Formed by Dispersing Pigment with Polymer (B)]

Examples of the configuration of the particles formed by dispersing thepigment with the polymer (B) include 1) particles formed by kneading thepigment and the polymer and then dispersing the resulting kneadedmaterial in a medium such as water; 2) particles formed by stirring thepigment and the polymer in a medium such as water to disperse thepigment in the medium such as water; 3) particles formed by mechanicallydispersing a polymer raw material and the pigment to polymerize thepolymer raw material and then dispersing the pigment in a medium such aswater with the resulting polymer; and the like.

In addition, from the viewpoint of improving storage stability of theparticles in the respective water-based inks, a crosslinking agent maybe added to the particles formed by dispersing the pigment with thepolymer to subject the polymer to crosslinking reaction.

[Polymer (B)]

As the polymer (B), there may be mentioned at least one polymer selectedfrom the group consisting of condensation-based resins such aspolyurethanes and polyesters, and vinyl-based polymers such as acrylicresins, styrene-based resins, styrene-acrylic resins, butadiene-basedresins, styrene-butadiene-based resins, vinyl chloride-based resins,vinyl acetate-based resins and acrylic-silicone-based resins. Amongthese polymers, preferred are vinyl-based polymers.

The weight-average molecular weight of the polymer (B) is preferably notless than 10,000, more preferably not less than 20,000, even morepreferably not less than 30,000 and further even more preferably notless than 40,000, and is also preferably not more than 2,500,000 andmore preferably not more than 1,000,000, from the viewpoint of improvingdispersibility of the pigment as well as from the viewpoint of improvingfixing properties of the resulting printed characters or images.

[Pigment Dispersing Polymer (B-1)]

As the pigment dispersing polymer (B-1), there may be mentioned at leastone polymer selected from the group consisting of condensation-basedresins such as polyesters and polyurethanes, and vinyl-based polymers,etc. Among these polymers, from the viewpoint of improving dispersionstability of the pigment, preferred are vinyl-based polymers obtained byaddition-polymerizing a vinyl monomer (such as vinyl compounds,vinylidene compounds and vinylene compounds). The pigment dispersingpolymer (B-1) used herein may be in the form of either an appropriatelysynthetized product or a commercially available product.

The weight-average molecular weight of the pigment dispersing polymer(B-1) is preferably not less than 20,000, more preferably not less than30,000 and even more preferably not less than 40,000, and is alsopreferably not more than 500,000, more preferably not more than 300,000and even more preferably not more than 200,000, from the viewpoint ofimproving dispersibility of the pigment.

Examples of the commercially available products of the vinyl-basedpolymers include polyacrylic acids such as “ARON AC-10SL” available fromToagosei Co., Ltd., and styrene-acrylic resins such as “JONCRYL 67”,“JONCRYL 611”, “JONCRYL 678”, “JONCRYL 680”, “JONCRYL 690” and “JONCRYL819” all available from BASF Japan, Ltd., etc.

[Fixing Aid Polymer (B-2)]

The fixing aid polymer (B-2) is preferably used in the form ofpigment-free polymer particles. The dispersion of the fixing aid polymer(B-2) serves for forming a film of the ink on a printing medium tothereby improve fixing properties of the ink.

As the fixing aid polymer (B-2), there may be mentioned at least onepolymer selected from the group consisting of condensation-based resinssuch as polyurethanes and polyesters, and vinyl-based polymers such asacrylic resins, styrene-based resins, styrene-acrylic resins,butadiene-based resins, styrene-butadiene-based resins, vinylchloride-based resins, vinyl acetate-based resins andacrylic-silicone-based resins. Among these polymers, from the viewpointof promoting drying of the ink on a printing medium and improving fixingproperties of the resulting printed characters or images, preferred areacrylic resins.

In addition, from the viewpoint of enhancing productivity of thewater-based inks, the fixing aid polymer (B-2) is preferably used in theform of a dispersion containing the polymer particles. As the fixing aidpolymer (B-2), there may be used either synthetized products obtained,for example, by emulsion polymerization method, etc., or commerciallyavailable products.

Examples of the commercially available products of the fixing aidpolymer (B-2) include acrylic resins such as “Neocryl A1127” (anionicself-crosslinkable aqueous acrylic resin) available from DSM NeoResins,Inc., and “JONCRYL 390” available from BASF Japan, Ltd.; urethane resinssuch as “WBR-2018” and “WBR-2000U” both available from Taisei FineChemical Co., Ltd.; styrene-butadiene resins such as “SR-100” and“SR-102” both available from Nippon A & L Inc.; styrene-acrylic resinssuch as “JONCRYL 7100”, “JONCRYL 7600”, “JONCRYL 537J”, “JONCRYL 538J”,“JONCRYL 780” and “JONCRYL PDX-7164” all available from BASF Japan,Ltd.; and vinyl chloride-based resins such as “VINYBLAN 700” and“VINYBLAN 701” both available from Nissin Chemical Co., Ltd., etc.

The fixing aid polymer (B-2) may be used in the form of particlesdispersed in water.

The weight-average molecular weight of the fixing aid polymer (B-2) ispreferably not less than 10,000, more preferably not less than 20,000and even more preferably not less than 30,000, and is also preferablynot more than 2,500,000 and more preferably not more than 1,000,000,from the viewpoint of improving fixing properties of the resulting ink.

In addition, the average particle size of particles of the fixing aidpolymer (B-2) in the dispersion containing the particles of the fixingaid polymer (B-2) or in the resulting ink is preferably not less than 10nm, more preferably not less than 30 nm and even more preferably notless than 50 nm, and is also preferably not more than 300 nm, morepreferably not more than 200 nm, even more preferably not more than 150nm and further even more preferably not more than 130 nm, from theviewpoint of improving storage stability of the resulting ink.

<Water-Insoluble Polymer (BX)>

The water-insoluble polymer (BX) is preferably used in the from ofpigment-containing water-insoluble polymer particles (hereinafter alsoreferred to merely as “pigment-containing polymer particles”) from theviewpoint of obtaining good printed characters or images which are freeof mottling or color bleeding. The term “water-insoluble” as used hereinmeans that the polymer has a solubility in water of not more than 10 g,preferably not more than 5 g and more preferably not more than 1 g whenthe polymer is dried to constant weight and then dissolved in 100 g ofwater at 25° C. In the case where the water-insoluble polymer (BX) is inthe form of an anionic polymer, the solubility thereof means asolubility of the polymer whose anionic groups are neutralizedcompletely (i.e., 100%) with NaOH.

As the water-insoluble polymer (BX), from the viewpoint of improvingstorage stability of the resulting ink, preferred is a vinyl-basedpolymer obtained by addition-polymerizing a vinyl monomer. Thevinyl-based polymer is preferably a vinyl-based polymer that is producedby copolymerizing a monomer mixture containing (b-1) an ionic monomerand (b-2) a hydrophobic monomer. The vinyl-based polymer contains aconstitutional unit derived from the component (b-1) and aconstitutional unit derived from the component (b-2). In particular, itis preferred that the vinyl-based polymer further contains aconstitutional unit derived from (b-3) a macromonomer.

Examples of the ionic monomer (b-1) include anionic monomers andcationic monomers. Among these monomers, preferred are anionic monomers.Examples of the anionic monomers include carboxylic acid monomers,sulfonic acid monomers and phosphoric acid monomers.

Specific examples of the carboxylic acid monomers include acrylic acid,methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaricacid and 2-methacryloyloxymethylsuccinic acid. Among the aforementionedanionic monomers, from the viewpoint of improving dispersion stabilityof the pigment-containing polymer particles in the ink, preferred arethe carboxylic acid monomers, and more preferred are acrylic acid andmethacrylic acid.

As the hydrophobic monomer (b-2), there may be mentioned at least onemonomer selected from the group consisting of alkyl (meth)acrylic acidesters and aromatic group-containing monomers.

The alkyl (meth)acrylic acid esters are preferably those alkyl(meth)acrylic acid esters containing an alkyl group having 1 to 22carbon atoms. Examples of the alkyl (meth)acrylic acid esters includemethyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate,butyl (meth)acrylate, amyl (meth)acrylate, cyclohexyl (meth)acrylate,2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate,dodecyl (meth)acrylate and (iso)stearyl (meth)acrylate.

The aromatic group-containing monomer is preferably a vinyl monomercontaining an aromatic group having 6 to 22 carbon atoms which maycontain a substituent group having a hetero atom, and more preferably astyrene-based monomer and an aromatic group-containing (meth)acrylicacid ester.

As the styrene-based monomer, preferred are styrene, 2-methyl styreneand divinyl benzene, and more preferred is styrene. As the aromaticgroup-containing (meth)acrylic acid ester, preferred are benzyl(meth)acrylate and phenoxyethyl (meth)acrylate, and more preferred isbenzyl (meth)acrylate.

The macromonomer (b-3) is in the form of a compound containing apolymerizable functional group at one terminal end thereof and having anumber-average molecular weight of not less than 500 and not more than100,000. The polymerizable functional group bonded to one terminal endof the macromonomer is preferably an acryloyloxy group or amethacryloyloxy group.

The macromonomer (b-3) preferably has a number-average molecular weightof not less than 1,000 and not more than 10,000. Meanwhile, thenumber-average molecular weight of the macromonomer (b-3) may bemeasured by gel permeation chromatography using polystyrene as areference standard substance.

As the macromonomer (b-3), from the viewpoint of improving dispersionstability of the pigment-containing polymer particles in the ink, thereare preferably used an aromatic group-containing monomer-basedmacromonomer and a silicone-based macromonomer. Among thesemacromonomers, more preferred is the aromatic group-containingmonomer-based macromonomer.

Examples of an aromatic group-containing monomer constituting thearomatic group-containing monomer-based macromonomer include thosearomatic group-containing monomers as described as to the aforementionedhydrophobic monomer (b-2). Among these aromatic group-containingmonomers, preferred are styrene and benzyl (meth)acrylate, and morepreferred is styrene.

Specific examples of the styrene-based macromonomer include “AS-6(S)”,“AN-6(S)” and “HS-6(S)” (tradenames) all available from Toagosei Co.,Ltd., etc.

From the viewpoint of improving dispersion stability of thepigment-containing polymer particles in the ink, it is preferred that(b-4) a nonionic monomer is further used as a monomer component of thewater-insoluble polymer (BX).

Examples of the component (b-4) include 2-hydroxyethyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, a polyalkylene glycol (meth)acrylatesuch as polypropylene glycol (n=2 to 30 wherein n represents an averagemolar number of addition of oxyalkylene groups: hereinafter defined inthe same way) (meth)acrylate and polyethylene glycol (n=2 to 30)(meth)acrylate, an alkoxy polyalkylene glycol (meth)acrylate such asmethoxy polyethylene glycol (n=1 to 30) (meth)acrylate, and phenoxy(ethylene glycol/propylene glycol copolymer) (n=1 to 30 in which n forethylene glycol: 1 to 29) (meth)acrylate.

Specific examples of commercially available products of the component(b-4) include “NK ESTER M-20G”, “NK ESTER M-40G”, “NK ESTER M-90G”, “NKESTER M-230G” and the like all available from Shin-Nakamura ChemicalCo., Ltd.; and “BLEMMER PE-90”, “BLEMMER PE-200”, “BLEMMER PE-350” andthe like, “BLEMMER PME-100”, “BLEMMER PME-200”, “BLEMMER PME-400” andthe like, “BLEMMER PP-500”, “BLEMMER PP-800”, “BLEMMER PP-1000” and thelike, “BLEMMER AP-150”, “BLEMMER AP-400”, “BLEMMER AP-550” and the like,and “BLEMMER 50PEP-300”, “BLEMMER 50POEP-800B”, “BLEMMER 43PAPE-600B”and the like all available from NOF Corporation.

The aforementioned components (b-1) to (b-4) may be respectively usedalone or in the form of a mixture of any two or more thereof.

The contents of the constitutional units derived from the components(b-1) to (b-4) in the water-insoluble polymer (BX) are as follows, fromthe viewpoint of improving dispersion stability of thepigment-containing polymer particles in the resulting ink.

The content of the component (b-1) is preferably not less than 3% bymass, more preferably not less than 5% by mass and even more preferablynot less than 7% by mass, and is also preferably not more than 40% bymass, more preferably not more than 30% by mass and even more preferablynot more than 20% by mass.

The content of the component (b-2) is preferably not less than 25% bymass, more preferably not less than 30% by mass and even more preferablynot less than 35% by mass, and is also preferably not more than 60% bymass, more preferably not more than 55% by mass and even more preferablynot more than 50% by mass.

The content of the component (b-3) is not less than 0% by mass,preferably not less than 5% by mass, more preferably not less than 8% bymass and even more preferably not less than 10% by mass, and is alsopreferably not more than 30% by mass, more preferably not more than 25%by mass and even more preferably not more than 20% by mass.

The content of the component (b-4) is not less than 0% by mass,preferably not less than 5% by mass, more preferably not less than 10%by mass and even more preferably not less than 15% by mass, and is alsopreferably not more than 50% by mass, more preferably not more than 45%by mass and even more preferably not more than 40% by mass.

[Production of Water-Insoluble Polymer (BX)]

The water-soluble polymer (BX) may be produced by copolymerizing theaforementioned monomers by known polymerization methods, for example, bya solution polymerization method. The water-insoluble polymer (BX) thusproduced is preferably used as such in the form of a water-insolublepolymer solution without removing the solvent used in the polymerizationreaction therefrom in order to use the organic solvent contained thereinas an organic solvent in the below-mentioned step 1, from the viewpointof enhancing productivity of the below-mentioned water dispersion of thepigment-containing polymer particles.

The weight-average molecular weight of the water-insoluble polymer (BX)is preferably not less than 5,000, more preferably not less than 10,000and even more preferably not less than 20,000, and is also preferablynot more than 500,000, more preferably not more than 400,000, even morepreferably not more than 300,000 and further even more preferably notmore than 200,000, from the viewpoint of obtaining good printedcharacters or images which are free of mottling or color bleeding.

Examples of the configuration of the water-insoluble polymer (BX)present in the respective water-based inks include the particleconfiguration in which the pigment is enclosed or encapsulated in thepolymer, the particle configuration in which the pigment is uniformlydispersed in the polymer, the particle configuration in which thepigment is exposed onto a surface of the respective polymer particles,the configuration in which the polymer is adsorbed onto the pigment, andthe configuration in which the polymer is unadsorbed onto the pigment,as well as mixtures of these configurations. Among these configurations,from the viewpoint of improving dispersion stability of the pigment, inthe present invention, the configuration in which the water-insolublepolymer (BX) is in the form of pigment-containing polymer particles ispreferably used, and the pigment-enclosing configuration in which thepigment is incorporated into the water-insoluble polymer (BX) is morepreferably used.

[Production of Particles of Pigment-Containing Water-Insoluble Polymer(BX) (Pigment-Containing Polymer Particles)]

The pigment-containing polymer particles can be efficiently produced inthe form of a water dispersion thereof by the process including thefollowing steps I and II, and further including the following step IIIas an optional step.

Step I; subjecting a mixture of the water-insoluble polymer (BX), anorganic solvent, a pigment and water (hereinafter also referred tomerely as a “pigment mixture”) to dispersion treatment to obtain adispersion of pigment-containing polymer particles;

Step II; removing the organic solvent from the dispersion obtained inthe step I to obtain a water dispersion of the pigment-containingpolymer particles (hereinafter also referred to merely as a “pigmentwater dispersion”); and

Step III; mixing the water dispersion obtained in the step II with acrosslinking agent to subject the polymer to crosslinking treatment,thereby obtaining a water dispersion of the crosslinked polymer.

(Step I)

In the step I, there is preferably used the method in which thewater-insoluble polymer (BX) is first dissolved in the organic solvent,and then the pigment and water, if required, together with aneutralizing agent, a surfactant and the like, are added and mixed inthe resulting organic solvent solution to obtain a dispersion of anoil-in-water type.

The organic solvent in which the water-insoluble polymer (BX) can bedissolved is not particularly limited. When the water-insoluble polymeris synthesized by the solution polymerization method, the solvent usedin the polymerization method may be directly used as such in the step I.

In the case where the water-insoluble polymer (BX) is an anionicpolymer, an anionic group contained in the water-insoluble polymer maybe neutralized using a neutralizing agent. Examples of the neutralizingagent include hydroxides of alkali metals, ammonia and organic amines.

The method of conducting the dispersion treatment used in the step I isnot particularly limited. It is preferred that the pigment mixture isfirst subjected to preliminary dispersion treatment, and then tosubstantial dispersion treatment by applying a shear stress thereto soas to control the average particle size of the obtained pigmentparticles to a desired value. Examples of the disperser used in thepreliminary dispersion treatment include ordinary mixing and stirringdevices such as anchor blades and disper blades. Of these devices,preferred are high-speed stirring mixers.

Examples of the disperser used in the substantial dispersion treatmentinclude kneading machines such as roll mills and kneaders, high-pressurehomogenizers such as “MICROFLUIDIZER” available from MicrofluidicsCorporation, and media-type dispersers such as paint shakers and beadsmills. Among these devices, from the viewpoint of reducing a particlesize of the pigment, the high-pressure homogenizers are preferably used.In the case where the substantial dispersion treatment is conductedusing the high-pressure homogenizer, the particle size of the pigmentcan be adjusted to a desired value by controlling the treating pressureand the number of passes through the homogenizer.

(Step II)

In the step II, the organic solvent is removed from the dispersionobtained in the step I by any conventionally known methods to obtain apigment water dispersion. The organic solvent is preferablysubstantially completely removed from the thus obtained pigment waterdispersion. However, the residual organic solvent may be present in thepigment water dispersion unless the objects and advantageous effects ofthe present invention are adversely affected by the residual organicsolvent.

In the thus obtained pigment water dispersion, the water-insolublepolymer (BX) in the form of solid particles containing the pigment isdispersed in a medium containing water as a main medium. Theconfiguration of the pigment-containing polymer particles in the pigmentwater dispersion is not particularly limited, and the pigment-enclosingconfiguration in which the pigment is incorporated into the polymer ispreferred as described previously.

(Step III)

The step III is an optional step. The step III is preferably carried outfrom the viewpoint of improving storage stability of the pigment waterdispersion and the resulting ink.

In the case where the water-insoluble polymer (BX) is an anionicwater-insoluble polymer containing an anionic group, the crosslinkingagent used in the step III is preferably in the form of a compoundcontaining a functional group that is capable of reacting with theanionic group of the polymer, more preferably a compound containing thetwo or more functional groups in a molecule thereof and even morepreferably a compound containing the 2 to 6 functional groups in amolecule thereof.

Suitable examples of the crosslinking agent include compounds containingtwo or more epoxy groups in a molecule thereof, compounds containing twoor more oxazoline groups in a molecule thereof, and compounds containingtwo or more isocyanate groups in a molecule thereof. Among thesecrosslinking agents, preferred are compounds containing two or moreepoxy groups in a molecule thereof, and more preferred istrimethylolpropane polyglycidyl ether.

The solid content of the resulting pigment water dispersion ispreferably not less than 10% by mass and more preferably not less than15% by mass, and is also preferably not more than 35% by mass and morepreferably not more than 30% by mass, from the viewpoint of improvingdispersion stability of the pigment water dispersion. Meanwhile, thesolid content of the pigment water dispersion may be measured by themethod described in Examples below.

The average particle size of the pigment-containing polymer particles inthe pigment water dispersion is preferably not less than 40 nm, morepreferably not less than 60 nm, even more preferably not less than 80 nmand further even more preferably not less than 85 nm, and is alsopreferably not more than 150 nm, more preferably not more than 130 nmand even more preferably not more than 125 nm, from the viewpoints ofsuppressing formation of coarse particles and improving continuousejection properties of the resulting ink.

Meanwhile, the average particle size of the pigment-containing polymerparticles may be measured by the method described in Examples below.

<Organic Solvent (C)>

As the organic solvent (C), those organic solvents having a boilingpoint of not lower than 90° C. and lower than 250° C. are preferred fromthe viewpoint of obtaining good printed characters or images which arefree of mottling or color bleeding. The boiling point of the organicsolvent (C) is more preferably not lower than 130° C., even morepreferably not lower than 140° C. and further even more preferably notlower than 150° C., and is also more preferably not higher than 245° C.,even more preferably not higher than 240° C. and further even morepreferably not higher than 235° C.

Examples of the suitable organic solvent (C) include a polyhydricalcohol (c-1) and a glycol ether (c-2).

(Polyhydric Alcohol (c-1))

Examples of the aforementioned polyhydric alcohol (c-1) include1,2-alkanediols such as ethylene glycol, propylene glycol,1,2-butanediol, 1,2-pentanediol and 1,2-hexanediol, diethylene glycol,polyethylene glycol, dipropylene glycol, 1,3-propanediol,1,3-butanediol, 1,4-butanediol, 3-methyl-1,3-butanediol,1,5-pentanediol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol,1,2,4-butanetriol, 1,2,3-butanetriol and petriol.

Among these polyhydric alcohols, from the viewpoint of improving storagestability and continuous ejection properties of the resulting ink,preferred is at least one compound selected from the group consisting ofalkanediols having not less than 2 and not more than 6 carbon atoms,such as propylene glycol (boiling point (b.p.): 188° C.), diethyleneglycol (b.p.: 245° C.) and 1,2-hexanediol (b.p.: 223° C.), andpolypropylene glycols having a molecular weight of 500 to 1000, and morepreferred is at least one compound selected from the group consisting of1,2-alkanediols having not less than 3 and not more than 4 carbon atoms,such as propylene glycol and diethylene glycol, and the aforementionedpolypropylene glycols.

(Glycol Ether (c-2))

Specific examples of the glycol ether (c-2) include at least onecompound selected from the group consisting of alkylene glycol monoalkylethers and alkylene glycol dialkyl ethers. Among these glycol ethers,from the viewpoint of improving continuous ejection properties of theresulting ink as well as from the viewpoint of obtaining good printedcharacters or images which are free of mottling or color bleeding,preferred are alkylene glycol monoalkyl ethers. The number of carbonatoms in an alkyl group of the alkylene glycol monoalkyl ethers ispreferably not less than 1, more preferably not less than 2 and evenmore preferably not less than 3, and is also preferably not more than 6and more preferably not more than 4. The alkyl group of the alkyleneglycol monoalkyl ethers may be in the form of either a straight chain ora branched chain.

Specific examples of the alkylene glycol monoalkyl ethers include atleast one compound selected from the group consisting of ethylene glycolethyl ether, ethylene glycol isopropyl ether, ethylene glycol propylether, ethylene glycol butyl ether, diethylene glycol methyl ether,diethylene glycol ethyl ether, diethylene glycol isopropyl ether,diethylene glycol isobutyl ether, diethylene glycol butyl ether,triethylene glycol methyl ether, dipropylene glycol butyl ether,dipropylene glycol methyl ether and tripropylene glycol methyl ether.

Of these alkylene glycol monoalkyl ethers, preferred is at least onecompound selected from the group consisting of ethylene glycol isopropylether (b.p.: 144° C.), ethylene glycol propyl ether (b.p.: 151° C.),diethylene glycol methyl ether (b.p.: 194° C.), diethylene glycolisopropyl ether (b.p.: 207° C.), diethylene glycol isobutyl ether (b.p.:230° C.) and diethylene glycol butyl ether (b.p.: 230° C.), and morepreferred is at least one compound selected from the group consisting ofethylene glycol isopropyl ether, diethylene glycol isopropyl ether anddiethylene glycol isobutyl ether.

(Other Organic Solvents)

In the present invention, the respective water-based inks may alsocontain, in addition to the aforementioned organic solvent (C), thoseorganic solvents that may be usually compounded in the water-based inks,such as the other alcohols, alkyl ethers of the alcohols, glycol ethers,nitrogen-containing heterocyclic compounds such as NMP, amides, aminesand sulfur-containing compounds.

For example, 1,6-hexanediol (b.p.: 250° C.), triethylene glycol (b.p.:285° C.), tripropylene glycol (b.p.: 273° C.), polypropylene glycol(b.p.: not lower than 250° C.) and glycerin (b.p.: 290° C.), etc., maybe used in combination with the aforementioned organic solvent having aboiling point of lower than 250° C.

<Surfactant (D)>

The respective water-based inks used in the present invention preferablyfurther contain a surfactant (D) from the viewpoint of obtaining goodprinted characters or images which are free of mottling or colorbleeding. As the surfactant (D), there are preferably used thosesurfactants including a silicone-based surfactant (d-1).

Any suitable silicone-based surfactants may be appropriately selectedand used as the silicone-based surfactant (d-1) according to the objectsand applications of the water-based inks. Among these silicone-basedsurfactants, from the viewpoint of suppressing increase in viscosity ofthe resulting inks, improving continuous ejection properties of the inksand obtaining good printed characters or images which are free ofmottling or color bleeding, a polyether-modified silicone-basedsurfactant is preferably used.

(Polyether-Modified Silicone-Based Surfactant)

The polyether-modified silicone-based surfactant is capable ofsuppressing increase in viscosity of the resulting inks and occurrenceof color bleeding between the inks. Therefore, it is considered that thepolyether-modified silicone-based surfactant contributes to productionof good printed materials that are free of mottling or color bleedingupon high-speed printing.

The polyether-modified silicone-based surfactant has such a structurethat a hydrocarbon group bonded to a side chain and/or a terminal end ofa silicone oil is substituted with a polyether group. Examples of thesuitable polyether group of the polyether-modified silicone-basedsurfactant include a polyethyleneoxy group, a polypropyleneoxy group anda polyalkyleneoxy group formed by addition-bonding an ethyleneoxy group(EO) and a propyleneoxy group (PO) to each other in a block form or arandom form. More specifically, as the polyether-modified silicone-basedsurfactant, there may be used a compound formed by grafting a polyethergroup to a main chain of a silicone, a compound formed by bonding asilicone and a polyether group to each other in a block form, etc.

The HLB value of the polyether-modified silicone-based surfactant ispreferably not less than 3.0, more preferably not less than 4.0 and evenmore preferably not less than 4.5 from the viewpoint of improvingsolubility of the polyether-modified silicone-based surfactant in therespective water-based inks. The term “HLB” as used herein means thevalue indicating an affinity of the surfactant to water and an oil, andcan be calculated according to the following formula by Griffin method.Meanwhile, as the “hydrophilic group contained in surfactant” shown inthe following formula, there may be mentioned, for example, a hydroxygroup and an ethyleneoxy group.HLB=20×[(molecular weight of hydrophilic group contained insurfactant)/(molecular weight of surfactant)]

Specific examples of the polyether-modified silicone-based surfactantinclude “KF” series products available from Shin-Etsu Chemical Co.,Ltd., “SILFACE SAG” available from Nissin Chemical Co., Ltd., and “BYK”series products available from BYK Chemie Japan K.K.

(Other Surfactants)

In the present invention, the polyether-modified silicone-basedsurfactant may be used in combination with the other surfactants. Amongthe surfactants other than the polyether-modified silicone-basedsurfactant, preferred is a nonionic surfactant.

Examples of the nonionic surfactant include (1) alkyl ethers and alkenylethers of polyoxyalkylenes which are produced by adding an alkyleneoxideto an aliphatic or aromatic alcohol having 8 to 22 carbon atoms or apolyhydric alcohol, (2) esters of an alcohol containing a hydrocarbongroup having 8 to 22 carbon atoms, and a polyvalent fatty acid, (3)polyoxyalkylene aliphatic amines containing an alkyl group or alkenylgroup having 8 to 20 carbon atoms, and (4) ester compounds of a higherfatty acid having 8 to 22 carbon atoms and a polyhydric alcohol, orcompounds produced by adding an alkyleneoxide to the ester compounds.

Examples of commercially available products of the nonionic surfactantinclude “SURFYNOL” series products available from Nissin Chemical Co.,Ltd., “ACETYLENOL” series products available from Kawaken Fine ChemicalsCo., Ltd., and “EMULGEN 120” (polyoxyethylene lauryl ether) availablefrom Kao Corporation.

[Contents of Respective Components in Water-Based Inks and Properties ofWater-Based Inks]

The respective water-based inks used in the present invention may beobtained by appropriately mixing the aforementioned components and thenstirring the resulting mixture. The contents of the respectivecomponents in the resulting water-based inks as well as variousproperties of the water-based inks are as follows.

(Content of Pigment (A))

The content of the pigment (A) in the respective water-based inks ispreferably not less than 2.0% by mass, more preferably not less than4.0% by mass and even more preferably not less than 6.0% by mass fromthe viewpoint of enhancing optical density of the water-based inks uponprinting. Also, the content of the pigment (A) in the respectivewater-based inks is preferably not more than 30.0% by mass, morepreferably not more than 20% by mass, even more preferably not more than15% by mass and further even more preferably not more than 10.0% by massfrom the viewpoint of reducing viscosity of the water-based inks uponvolatilization of the solvent therefrom as well as from the viewpoint ofimproving continuous ejection properties of the water-based inks andobtaining good printed characters or images which are free of mottlingor color bleeding.

(Content of Polymer (B))

The content of the polymer (B) in the respective water-based inks ispreferably not less than 1.0% by mass, more preferably not less than2.0% by mass and even more preferably not less than 3.0% by mass, and isalso preferably not more than 20% by mass, more preferably not more than13% by mass and even more preferably not more than 8.0% by mass, fromthe viewpoint of improving fixing properties of the water-based inks.

In the case where the polymer (B) is used as the pigment dispersingpolymer (B-1), the content of the pigment dispersing polymer (B-1) inthe respective water-based inks is preferably not less than 0.01% bymass, more preferably not less than 0.05% by mass and even morepreferably not less than 0.1% by mass, and is also preferably not morethan 10% by mass, more preferably not more than 7.0% by mass and evenmore preferably not more than 5.0% by mass, from the viewpoint ofimproving fixing properties of the water-based inks.

In the case where the polymer (B) is used as the fixing aid polymer(B-2), the content of the fixing aid polymer (B-2) in the respectivewater-based inks is preferably not less than 0.9% by mass, morepreferably not less than 1.0% by mass and even more preferably not lessthan 1.2% by mass, and is also preferably not more than 10% by mass,more preferably not more than 6.0% by mass and even more preferably notmore than 3.0% by mass, from the viewpoint of improving fixingproperties of the water-based inks.

Meanwhile, when using the particles of the pigment-containingwater-insoluble polymer (BX), the content of the polymer (B) in therespective water-based inks as used herein means a total content of thepigment dispersing polymer (B-1) of the pigment-containing polymerparticles and the fixing aid polymer (B-2).

(Content of Organic Solvent (C))

The content of the organic solvent (C) in the respective water-basedinks is preferably not less than 15% by mass, more preferably not lessthan 20% by mass and even more preferably not less than 25% by mass, andis also preferably not more than 45% by mass, more preferably not morethan 40% by mass and even more preferably not more than 35% by mass,from the viewpoint of improving continuous ejection properties of theresulting water-based inks.

The content of the polyhydric alcohol (c-1) in the respectivewater-based inks is preferably not less than 10% by mass, morepreferably not less than 15% by mass and even more preferably not lessthan 20% by mass, and is also preferably not more than 45% by mass, morepreferably not more than 40% by mass and even more preferably not morethan 35% by mass, from the viewpoint of improving storage stability andcontinuous ejection properties of the resulting water-based inks.

The content of the glycol ether (c-2) in the respective water-based inksis preferably not less than 1% by mass, more preferably not less than 2%by mass and even more preferably not less than 3% by mass, and is alsopreferably not more than 15% by mass, more preferably not more than 12%by mass and even more preferably not more than 8% by mass, from theviewpoint of improving storage stability and continuous ejectionproperties of the resulting water-based inks.

The content of a high-boiling organic solvent having a boiling point ofnot lower than 250° C. in the respective water-based inks used in thepresent invention is preferably not more than 5% by mass, morepreferably not more than 4% by mass and even more preferably not morethan 3% by mass from the viewpoint of imparting adequate dryingproperties to the water-based inks upon high-speed printing andobtaining good printed characters or images which are free of mottlingor color bleeding.

(Content of Surfactant (D))

The content of the surfactant (D) in the respective water-based inks ispreferably not less than 0.1% by mass, more preferably not less than0.2% by mass and even more preferably not less than 0.5% by mass, and isalso preferably not more than 5% by mass, more preferably not more than4% by mass, even more preferably not more than 3% by mass and furthereven more preferably not more than 2.5% by mass, from the viewpoint ofsuppressing increase in viscosity of the water-based inks and improvingcontinuous ejection properties of the water-based inks as well as fromthe viewpoint of obtaining good printed characters or images which arefree of mottling or color bleeding.

The content of the silicone-based surfactant (d-1) in the respectivewater-based inks is preferably not less than 0.005% by mass, morepreferably not less than 0.03% by mass and even more preferably not lessthan 0.04% by mass, and is also preferably not more than 0.5% by mass,more preferably not more than 0.4% by mass and even more preferably notmore than 0.3% by mass, from the same viewpoint as described above.

The content of the polyether-modified silicone-based surfactant in therespective water-based inks is preferably not less than 0.01% by mass,more preferably not less than 0.02% by mass and even more preferably notless than 0.03% by mass, and is also preferably not more than 0.5% bymass, more preferably not more than 0.4% by mass and even morepreferably not more than 0.2% by mass, from the same viewpoint asdescribed above.

The content of the nonionic surfactant (d-2) in the respectivewater-based inks is preferably not less than 0.1% by mass, morepreferably not less than 0.8% by mass and even more preferably not lessthan 1.5% by mass, and is also preferably not more than 5% by mass, morepreferably not more than 3.5% by mass and even more preferably not morethan 2.5% by mass, from the same viewpoint as described above.

(Content of Water)

The content of water in the respective water-based inks is preferablynot less than 20% by mass, more preferably not less than 30% by mass andeven more preferably not less than 40% by mass, and is also preferablynot more than 85% by mass, more preferably not more than 80% by mass andeven more preferably not more than 75% by mass, from the viewpoint ofimproving continuous ejection properties and storage stability of thewater-based inks as well as from the viewpoint of obtaining good printedcharacters or images which are free of mottling or color bleeding.

The respective water-based inks used in the present invention may alsocontain, in addition to the aforementioned components, various ordinaryadditives such as a humectant, a wetting agent, a penetrant, a defoamingagent, an antiseptic agent, a mildew-proof agent and a rust preventive.

The respective water-based inks used in the present invention preferablycontain no radical-polymerizable compound from the viewpoint ofimproving safety and reducing occurrence of unpleasant smell uponprinting characters or images on food packages, etc.

<Properties of Water-Based Inks>

In the ink-jet printing method of the present invention, there are usedtwo or more kinds of water-based inks which are selected from the groupconsisting of a black ink, a white ink and chromatic inks and aredifferent in static surface tension from each other. The water-basedinks are preferably constituted of three or more kinds of water-basedinks, and more preferably four or more kinds of water-based inks, andare also preferably constituted of eight or less kinds of water-basedinks, and more preferably seven or less kinds of water-based inks.Examples of a combination of a plurality of the water-based inks includethe combination of four kinds of water-based inks including a black ink,a cyan ink, a magenta ink and a yellow ink, the combination of fivekinds of water-based inks including a black ink, a cyan ink, a magentaink, a yellow ink and a white ink, the combination of seven kinds ofwater-based inks including a black ink, a cyan ink, a magenta ink, ayellow ink, a red ink, a green ink and a blue ink, and the combinationof eight kinds of water-based inks including a black ink, a cyan ink, amagenta ink, a yellow ink, a red ink, a green ink, a blue ink and awhite ink.

From the viewpoint of obtaining good printed characters or images whichare free of mottling or color bleeding as well as from the viewpoint ofimproving ejection properties of the water-based inks, the staticsurface tension of the respective water-based inks as measured at 20° C.is preferably not less than 22 mN/m, more preferably not less than 24mN/m and even more preferably not less than 25 mN/m, and is alsopreferably not more than 45 mN/m, more preferably not more than 40 mN/mand even more preferably not more than 35 mN/m.

The difference in static surface tension between the aforementioned twoor more kinds of water-based inks is preferably not less than 0.1 mN/m,more preferably not less than 0.2 mN/m and even more preferably not lessthan 0.3 mN/m from the viewpoint of obtaining good printed characters orimages which are free of mottling or color bleeding.

The static surface tension of the respective inks may be adequatelycontrolled, for example, by suitably selecting the kind or content ofthe organic solvent (C) or the surfactant (D).

The average particle size of the particles contained in the respectivewater-based inks is preferably not less than 40 nm, more preferably notless than 60 nm and even more preferably not less than 80 nm, and isalso preferably not more than 250 nm, more preferably not more than 220nm, even more preferably not more than 200 nm and further even morepreferably not more than 180 nm, from the viewpoint of improving storagestability and ejection properties of the water-based inks.

Meanwhile, the average particle size and static surface tension of therespective water-based inks may be measured by the methods described inExamples below.

The viscosity of the respective water-based inks as measured at 32° C.is preferably not less than 2.0 mPa·s, more preferably not less than 3.0mPa·s and even more preferably not less than 4.0 mPa·s, and is alsopreferably not more than 12 mPa·s, more preferably not more than 9.0mPa·s and even more preferably not more than 7.0 mPa·s, from theviewpoint of improving continuous ejection properties of the water-basedinks.

The pH value of the respective water-based inks is preferably not lessthan 7.0, more preferably not less than 8.0, even more preferably notless than 8.5 and further even more preferably not less than 8.7 fromthe viewpoint of improving storage stability and the like of thewater-based inks as well as from the viewpoint of obtaining good printedcharacters or images which are free of mottling or color bleeding, andis also preferably not more than 11.0 and more preferably not more than10.0 from the viewpoint of improving resistance of members to thewater-based inks and suppressing skin irritation.

<Ink-Jet Printing Method>

The ink-jet printing method of the present invention is the method ofprinting characters or images on a low-liquid absorbing printing mediumusing an ink-jet printing apparatus equipped with an ink-jet print headthat is capable of ejecting the aforementioned water-based inks onto thelow-liquid absorbing printing medium, in which the ink-jet printingapparatus is loaded with the two or more kinds of water-based inks whichare different in static surface tension from each other; the printingmedium is heated to a temperature of from 30 to 75° C.; and thewater-based inks are ejected in the sequential order from thewater-based ink having a higher static surface tension.

Thus, in the present invention, by adjusting the static surface tensionof the subsequently ejected ink in advance so as to be lower than thatof the precedingly ejected ink, it is possible to reduce a difference instatic surface tension between the respective color inks during theheating and drying step, and therefore it is possible to suppressoccurrence of mottling or color bleeding.

In the present invention, the colorant-containing inks are preferablyejected onto the printing medium that is not subjected to anypretreatments such as application of a treating solution onto theprinting medium, to directly print characters or images thereon.

(Low-Liquid Absorbing Printing Medium)

Examples of the low-liquid absorbing printing medium used in the presentinvention include a low-liquid absorbing coated paper and a low-liquidabsorbing resin film. The low-liquid absorbing printing medium may be inthe form of either a sheet of paper or a roll of paper. However, fromthe viewpoint of enhancing productivity of printed materials, aroll-shaped printing medium is preferably used.

Examples of the coated paper include a general-purpose glossy paper “OKTopcoat Plus” (water absorption as measured in a pure water contact timeof 100 milliseconds (hereinafter defined in the same way): 4.9 g/m²)available from Oji Paper Co., Ltd., a multi-color foam glossy coatedpaper (water absorption: 5.2 g/m²) available from Oji Paper Co., Ltd.,“UPM Finesse Gloss” (water absorption: 3.1 g/m²) available from UPM, andthe like.

As the resin film, there may be used a transparent synthetic resin film.Examples of the transparent synthetic resin film include a polyesterfilm, a polyvinyl chloride film, a polypropylene film, a polyethylenefilm and a nylon film. These resin films may be in the from of either abiaxially oriented film, a monoaxially oriented film or a non-orientedfilm. Among these films, preferred are a polyester film and an orientedpolypropylene film, and more preferred are a polyester film such as apolyethylene terephthalate (PET) film subjected to corona dischargetreatment, and an oriented polypropylene film such as a biaxiallyoriented polypropylene (OPP) film subjected to corona dischargetreatment.

The thickness of the resin film is not particularly limited, and theresin film may be in the form of a thin film having a thickness of notless than 1 μm and less than 20 μm. However, the thickness of the resinfilm is preferably not less than 20 μm, more preferably not less than 30μm and even more preferably not less than 35 μm, and is also preferablynot more than 100 μm, more preferably not more than 80 μm and even morepreferably not more than 75 μm, from the viewpoint of suppressingdeterioration in appearance of the printing medium as well as from theviewpoint of improving availability of the printing medium.

Examples of commercially available products of the transparent syntheticresin film include “LUMIRROR T60” (PET) available from Toray Industries,Inc., “TAIKO FE2001” (corona discharge-treated PET) available fromFutamura Chemical Co, Ltd., “TAIKO FOR-AQ” (corona discharge-treatedOPP) available from Futamura Chemical Co, Ltd., “PVC80B P” (polyvinylchloride) available from Lintec Corporation, “KINATH KEE 70CA”(polyethylene) available from Lintec Corporation, “YUPO SG90 PAT1” (PP)available from Lintec Corporation and “BONYL RX” (nylon) available fromKohjin Film & Chemicals Co., Ltd., etc.

(Ink-Jet Printing Apparatus)

The ink-jet printing apparatus used in the present invention is loadedwith the two or more kinds of water-based inks. The structure in whichthe ink-jet printing apparatus is loaded with a plurality of thewater-based inks means that an ink-jet print head of the ink-jetprinting apparatus is provided with discharge ports through which aplurality of the water-based inks can be respectively ejected. Forexample, in the case where the ink-jet printing apparatus is loaded withthe two kinds of water-based inks, the ink-jet print head of the ink-jetprinting apparatus is provided with discharge ports through which thetwo kinds of water-based inks are respectively ejected. In this case,the number of the ink-jet print head(s) may be either 1 or 2. Thus, theink-jet printing apparatus is loaded with the two or more kinds ofwater-based inks. From the viewpoint of obtaining good printedcharacters or images which are free of mottling or color bleeding, theink-jet printing apparatus is preferably loaded with the three or morekinds of water-based inks, and more preferably loaded with the four ormore kinds of water-based inks, and is also preferably loaded with theeight or less kinds of water-based inks, and more preferably loaded withseven or less kinds of water-based inks.

The ink-jet printing apparatus used in the present invention ispreferably equipped with not only the ink-jet print head for ejectingthe water-based inks onto the printing medium that is transported in afeed direction thereof, but also an under heater for heating theprinting medium from a rear side surface of the printing medium which isopposed to a front side surface thereof which faces to the ink-jet printhead, for the purpose of heating the printing medium to a temperature of30 to 75° C.

(Ink-Jet Print Head)

As the ink-jet print head, there may be used a serial-type print headand a line-type print head, and the line-type print head is preferablyused in the present invention. In the ink-jet printing system using theline-type print head, while maintaining the print head in a stationerystate and moving the printing medium along a transportation directionthereof, droplets of the respective inks are ejected from openings ofnozzles of the print head in association with the movement of theprinting medium, whereby it is possible to allow the ink droplets toadhere onto the printing medium to print characters or images, etc., bya single-pass (one-pass) method thereon.

The ink droplets are preferably ejected by a piezoelectric method. Inthe piezoelectric method, the ink droplets are ejected from a number ofnozzles communicated with respective pressure chambers by vibrating awall surface of the respective pressure chambers by means of apiezoelectric element. Meanwhile, in the present invention, there mayalso be used a thermal method for ejecting the ink droplets.

The voltage applied to the print head is preferably not less than 5 V,more preferably not less than 10 V and even more preferably not lessthan 15 V, and is also preferably not more than 40 V, more preferablynot more than 35 V and even more preferably not more than 30 V, from theviewpoint of conducting the high-speed printing with a high efficiency,etc.

The drive frequency of the print head is preferably not less than 2 kHz,more preferably not less than 5 kHz and even more preferably not lessthan 8 kHz, and is also preferably not more than 80 kHz, more preferablynot more than 70 kHz and even more preferably not more than 60 kHz, fromthe viewpoint of conducting the high-speed printing with a highefficiency, etc.

(Printing Conditions, etc.)

The amount of the ink droplets ejected is preferably not less than 0.5pL, more preferably not less than 1.0 pL, even more preferably not lessthan 1.5 pL and further even more preferably not less than 1.8 pL, andis also preferably not more than 20 pL, more preferably not more than 15pL and even more preferably not more than 13 pL, as calculated per oneink droplet ejected, from the viewpoint of maintaining accuracy ofimpact positions of the ink droplets and improving quality of printedcharacters or images.

The print head resolution is preferably not less than 400 dpi(dot/inch), more preferably not less than 500 dpi and even morepreferably not less than 550 dpi.

From the viewpoint of reducing viscosity of the water-based inks andimproving continuous ejection properties of the water-based inks, theinside temperature of the print head, preferably a line-type print head,upon the printing, is preferably controlled to not lower than 20° C.,more preferably not lower than 25° C. and even more preferably not lowerthan 30° C., and is also preferably controlled to not higher than 45°C., more preferably not higher than 40° C. and even more preferably nothigher than 38° C.

The surface of the printing medium opposed to an ink-ejecting region ofthe print head, preferably a line-type print head, from which therespective inks are ejected, is heated to a temperature of not lowerthan 30° C. from the viewpoint of obtaining good printed characters orimages which are free of mottling or color bleeding, and is also heatedto a temperature of not higher than 75° C. from the viewpoint ofsuppressing thermal deformation of the printing medium and saving energyupon production of printed materials. The temperature of the printingmedium is preferably controlled to not lower than 35° C., morepreferably not lower than 40° C. and even more preferably not lower than45° C., and is also preferably controlled to not higher than 70° C.,more preferably not higher than 65° C., even more preferably not higherthan 60° C. and further even more preferably not higher than 55° C.

The temperature of the printing medium may be suitably controlled byadjusting a temperature of the under heater.

The printing speed is usually not less than 5 m/min, preferably not lessthan 10 m/min, more preferably not less than 20 m/min and even morepreferably not less than 30 m/min in terms of a transportation speed ofthe printing medium in the direction along which the printing medium ismoved upon the printing, from the viewpoint of enhancing productivity ofprinted materials, and is also preferably not more than 75 m/min interms of a transportation speed of the printing medium in the directionalong which the printing medium is moved upon the printing, from theviewpoint of improving operability of the printing apparatus.

The amount of the respective water-based inks deposited on the printingmedium is preferably not less than 0.1 g/m², and is also preferably notmore than 25 g/m² and more preferably not more than 20 g/m², in terms ofan amount of solid components therein, from the viewpoint of improvingquality of the printed characters or images and increasing the printingspeed.

(Under Heater)

The under heater mounted in the ink-jet printing apparatus is disposedon a rear side surface of the printing medium which is opposed to afront side surface thereof which faces to the print head for therespective water-based inks to heat the printing medium. The underheater may be, for example, in the form of a heater of a hot water typeor a heater of a thermoelectric type having a stainless steel or ceramicplate. In the case where a plurality of print heads capable of ejectinga plurality of inks are mounted in the ink-jet printing apparatus, it ispreferred that a plurality of under heaters are disposed atcorresponding positions opposed to the respective print heads.

The respective under heaters are disposed at a position spaced at apredetermined distance apart from the rear side surface of the printingmedium upon the printing. The predetermined distance is preferably notless than 0.05 mm, more preferably not less than 0.1 mm, even morepreferably not less than 0.2 mm and further even more preferably notless than 0.4 mm, and is also preferably not more than 5.0 mm, morepreferably not more than 4.0 mm, even more preferably not more than 3.0mm, further even more preferably not more than 2.0 mm and still furthereven more preferably not more than 1.5 mm.

In the ink-jet printing method of the present invention, in order toavoid occurrence of color bleeding between the respective ink dropletseven when ejecting a kind of ink onto a printing medium to printcharacters or images thereon and then successively ejecting a subsequentink on the printing medium, the ink-jet printing apparatus may befurther equipped with a fixing/curing means for applying a thermalenergy to the printing medium, such as a heater.

With respect to the aforementioned embodiments, the present inventionfurther provides the following aspects relating to the ink-jet printingmethod.

<1> An ink-jet printing method including the step of printing charactersor images on a low-liquid absorbing printing medium using an ink-jetprinting apparatus equipped with an ink-jet print head that is capableof ejecting water-based inks onto the low-liquid absorbing printingmedium, in which:

the water-based inks each contain a colorant (A), an organic solvent (C)and water;

the ink-jet printing apparatus is loaded with the two or more kinds ofwater-based inks which are different in static surface tension from eachother;

the printing medium is heated to a temperature of from 30 to 75° C.; and

the water-based inks are ejected in the sequential order from thewater-based ink having a higher static surface tension.

<2> The ink-jet printing method according to the aspect <1>, wherein thecolorant (A) in the respective water-based inks is present in the formof pigment-containing water-insoluble polymer particles.

<3> The ink-jet printing method according to the aspect <1> or <2>,wherein the organic solvent (C) contains a glycol ether (c-2).

<4> The ink-jet printing method according to any one of the aspects <1>to <3>, wherein a boiling point of the organic solvent (C) is preferablynot lower than 90° C., more preferably not lower than 130° C., even morepreferably not lower than 140° C. and further even more preferably notlower than 150° C., and is also preferably lower than 250° C., morepreferably not higher than 245° C., even more preferably not higher than240° C. and further even more preferably not higher than 235° C.<5> The ink-jet printing method according to any one of the aspects <1>to <4>, wherein a content of the organic solvent (C) in the respectivewater-based inks is preferably not less than 15% by mass, morepreferably not less than 20% by mass and even more preferably not lessthan 25% by mass, and is also preferably not more than 45% by mass, morepreferably not more than 40% by mass and even more preferably not morethan 35% by mass.<6> The ink-jet printing method according to any one of the aspects <1>to <5>, wherein the water-based inks each further contain a polymer (B).<7> The ink-jet printing method according to any one of the aspects <1>to <6>, wherein the polymer (B) is at least one polymer selected fromthe group consisting of condensation-based resins such as polyurethanesand polyesters, and vinyl-based polymers such as acrylic resins,styrene-based resins, styrene-acrylic resins, butadiene-based resins,styrene-butadiene-based resins, vinyl chloride-based resins, vinylacetate-based resins and acrylic-silicone-based resins, and preferably avinyl-based polymer.<8> The ink-jet printing method according to any one of the aspects <1>to <7>, wherein the polymer (B) is a pigment dispersing polymer (B-1)for dispersing the pigment, a fixing aid polymer (B-2) for improvingfixing properties of the printed characters or images, or a combinationof the pigment dispersing polymer (B-1) and the fixing aid polymer(B-2).<9> The ink-jet printing method according to any one of the aspects <1>to <8>, wherein the water-based inks each further contain a surfactant(D).<10> The ink-jet printing method according to the aspect <9>, whereinthe surfactant (D) contains a silicone-based surfactant (d-1).<11> The ink-jet printing method according to any one of the aspects <2>to <10>, wherein a content of the pigment (A) in the respectivewater-based inks is preferably not less than 2.0% by mass, morepreferably not less than 4.0% by mass and even more preferably not lessthan 6.0% by mass, and is also preferably not more than 30.0% by mass,more preferably not more than 20% by mass, even more preferably not morethan 15% by mass and further even more preferably not more than 10.0% bymass.<12> The ink-jet printing method according to any one of the aspects <6>to <11>, wherein a content of the polymer (B) in the respectivewater-based inks is preferably not less than 1.0% by mass, morepreferably not less than 2.0% by mass and even more preferably not lessthan 3.0% by mass, and is also preferably not more than 20% by mass,more preferably not more than 13% by mass and even more preferably notmore than 8.0% by mass.<13> The ink-jet printing method according to any one of the aspects <1>to <12>, wherein a content of the organic solvent (C) in the respectivewater-based inks is preferably not less than 15% by mass, morepreferably not less than 20% by mass and even more preferably not lessthan 25% by mass, and is also preferably not more than 45% by mass, morepreferably not more than 40% by mass and even more preferably not morethan 35% by mass.<14> The ink-jet printing method according to any one of the aspects <9>to <13>, wherein a content of the surfactant (D) in the respectivewater-based inks is preferably not less than 0.1% by mass, morepreferably not less than 0.2% by mass and even more preferably not lessthan 0.5% by mass, and is also preferably not more than 5% by mass, morepreferably not more than 4% by mass, even more preferably not more than3% by mass and further even more preferably not more than 2.5% by mass.<15> The ink-jet printing method according to any one of the aspects <1>to <14>, wherein a content of water in the respective water-based inksis preferably not less than 20% by mass, more preferably not less than30% by mass and even more preferably not less than 40% by mass, and isalso preferably not more than 85% by mass, more preferably not more than80% by mass and even more preferably not more than 75% by mass.<16> The ink-jet printing method according to any one of the aspects <1>to <15>, wherein the water-based inks preferably each contain noradical-polymerizable compound.<17> The ink-jet printing method according to any one of the aspects <1>to <16>, wherein a static surface tension of the respective water-basedinks as measured at 20° C. is preferably not less than 22 mN/m, morepreferably not less than 24 mN/m and even more preferably not less than25 mN/m, and is also preferably not more than 45 mN/m, more preferablynot more than 40 mN/m and even more preferably not more than 35 mN/m.<18> The ink-jet printing method according to any one of the aspects <1>to <17>, wherein a difference in static surface tension between the twoor more kinds of water-based inks is preferably not less than 0.1 mN/m,more preferably not less than 0.2 mN/m and even more preferably not lessthan 0.3 mN/m.<19> The ink-jet printing method according to any one of the aspects <1>to <18>, wherein a viscosity of the respective water-based inks asmeasured at 32° C. is preferably not less than 2.0 mPa·s, morepreferably not less than 3.0 mPa·s and even more preferably not lessthan 4.0 mPa·s, and is also preferably not more than 12 mPa·s, morepreferably not more than 9.0 mPa·s and even more preferably not morethan 7.0 mPa·s.<20> The ink-jet printing method according to any one of the aspects <1>to <19>, wherein the low-liquid absorbing printing medium is a polyesterfilm subjected to corona discharge treatment, or an orientedpolypropylene film subjected to corona discharge treatment.<21> The ink-jet printing method according to any one of the aspects <1>to <20>, wherein the respective colorant-containing inks are ejectedonto the printing medium that is not subjected to any pretreatments.<22> The ink-jet printing method according to any one of the aspects <1>to <21>, wherein the printing medium is heated by the ink-jet print headthat is capable of ejecting the water-based inks onto the printingmedium and an under heater that is disposed on a rear side surface ofthe printing medium opposed to a front side surface thereof which facesto the ink-jet print head.<23> The ink-jet printing method according to any one of the aspects <1>to <22>, wherein a printing speed is not less than 5 m/min and not morethan 75 m/min in terms of a transportation speed of the printing medium.<24> The ink-jet printing method according to any one of the aspects <1>to <23>, wherein the characters or images are printed on the printingmedium by a single-pass (one-pass) method.

EXAMPLES

In the following Production Examples, Examples and Comparative Examples,the “part(s)” and “%” indicate “part(s) by mass” and “% by mass”,respectively, unless otherwise specified. Meanwhile, the methods ofmeasuring respective properties and characteristics are as follows.

(1) Measurement of Weight-Average Molecular Weight of Water-InsolublePolymer

The weight-average molecular weight of the polymer was measured by gelpermeation chromatography [GPC apparatus: “HLC-8120GPC” available fromTosoh Corporation; columns: “TSK-GEL, α-M”×2 available from TosohCorporation; flow rate: 1 mL/min] using a solution prepared bydissolving phosphoric acid and lithium bromide in N,N-dimethyl formamidesuch that the concentrations of phosphoric acid and lithium bromide inthe solution were 60 mmol/L and 50 mmol/L, respectively, as an eluent,and using monodisperse polystyrenes having previously known molecularweights as a reference standard substance.

(2) Measurement of Solid Content of Pigment Water Dispersion

Sodium sulfate dried to constant weight in a desiccator was weighed inan amount of 10.0 g and charged into a 30 mL polypropylene container (ϕ:40 mm; height: 30 mm), and about 1.0 g of a sample to be measured wasadded to the container. The contents of the container were mixed witheach other and then accurately weighed. The resulting mixture wasmaintained in the container at 105° C. for 2 hours to remove volatilecomponents therefrom, and further allowed to stand in a desiccator for15 minutes to measure a mass thereof. The mass of the sample afterremoving the volatile components therefrom was regarded as a mass ofsolids therein. The solid content of the sample was calculated bydividing the mass of the solids by the mass of the sample initiallyadded.

(3) Measurement of Average Particle Sizes of Pigment-Containing PolymerParticles and Polymer Particles

The particles were subjected to cumulant analysis using a laser particleanalyzing system “ELS-8000” available from Otsuka Electrics Co., Ltd.,to measure an average particle size thereof. The above measurement wasconducted under the conditions including a temperature of 25° C., anangle between incident light and detector of 90° and a cumulative numberof 100 times, and a refractive index of water (1.333) was input to theanalyzing system as a refractive index of the dispersing medium. Themeasurement was also conducted by adjusting a concentration of thedispersion to be measured to 5×10⁻³% by mass in terms of a solid contentthereof.

(4) Measurement of Viscosity of Water-Based Ink

The viscosity of the water-based ink was measured at 32° C. using anE-type viscometer “TV-25” (equipped with a standard cone rotor (1°34′×R24); rotating speed: 50 rpm) available from Toki Sangyo Co., Ltd.

(5) Measurement of Static Surface Tension of Water-Based Ink

A platinum plate was dipped in 5 g of the water-based ink filled in acylindrical polyethylene container (3.6 cm in diameter×1.2 cm in depth),and the static surface tension of the water-based ink was measured at20° C. using a surface tension meter “CBVP-Z” (tradename) available fromKyowa Interface Science Co., Ltd.

(6) Measurement of pH of Water-Based Ink

The pH value of the water-based ink was measured at 25° C. using abench-top pH meter “F-71” available from Horiba Ltd., equipped with a pHelectrode “6337-10D” available from Horiba Ltd.

(7) Water Absorption of Printing Medium as Measured in Pure WaterContact Time of 100 Milliseconds

Using an automatic scanning absorptometer “KM500win” available fromKumagai Riki Kogyo Co., Ltd., the amount of pure water transferred to aprinting medium when contacted with pure water for 100 milliseconds wasmeasured at 23° C. under a relative humidity of 50%. The thus measuredamount of pure water transferred to the printing medium was determinedas a water absorption of the printing medium as measured in a pure watercontact time of 100 milliseconds. The measuring conditions are shownbelow.

“Spiral Method” Contact time: 0.010 to 1.0 (sec) Pitch (mm): 7 LengthPer Sampling (degree): 86.29 Start Radius (mm): 20 End Radius (mm): 60Min Contact Time (ms): 10 Max Contact Time (ms): 1000 Sampling Pattern(1-50): 50 Number of Sampling Points (>0): 19 “Square Head” Split Span(mm): 1 Split Width (mm): 5

Production Example 1 (Synthesis of Water-Insoluble Polymer)

Sixteen (16) parts of methacrylic acid available from Wako Pure ChemicalIndustries, Ltd., 44 parts of styrene available from Wako Pure ChemicalIndustries, Ltd., 30 parts of a styrene macromonomer “AS-6S”(number-average molecular weight: 6,000; solid content: 50%) availablefrom Toagosei Co., Ltd., and 25 parts of methoxypolyethylene glycolmethacrylate “BLEMMER PME-200” available from NOF Corporation were mixedwith each other to prepare 115 parts of a monomer mixture solution.

Eighteen (18) parts of methyl ethyl ketone and 0.03 part of2-mercaptoethanol as a chain transfer agent as well as 10% (11.5 parts)of the monomer mixture solution prepared above were charged into areaction vessel and mixed with each other, and then an inside atmosphereof the reaction vessel was fully replaced with nitrogen gas.

Separately, a mixed solution prepared by mixing remaining 90% (103.5parts) of the monomer mixture solution, 0.27 part of the aforementionedchain transfer agent, 42 parts of methyl ethyl ketone and 3 parts of2,2′-azobis(2,4-dimethylvaleronitrile) “V-65” as a polymerizationinitiator available from Wako Pure Chemical Industries, Ltd., wascharged into a dropping funnel. In a nitrogen atmosphere, the mixedsolution in the reaction vessel was heated to 75° C. while stirring, andthen the mixed solution in the dropping funnel was added dropwisethereinto over 3 hours. After the elapse of 2 hours from completion ofthe dropwise addition while maintaining the resulting mixed solution ata temperature of 75° C., a solution prepared by dissolving 3 parts ofthe aforementioned polymerization initiator in 5 parts of methyl ethylketone was added to the mixed solution, and the resulting reactionsolution was further aged at 75° C. for 2 hours and then at 80° C. for 2hours, followed by further adding 50 parts of methyl ethyl ketonethereto, thereby obtaining a solution of a water-insoluble polymer(having a weight-average molecular weight of 50,000). The solid contentof the thus obtained water-insoluble polymer solution was 45% by mass.

Production Example 2 (Production of Water Dispersion of BlackPigment-Containing Polymer Particles)

Added into a solution prepared by dissolving 95.2 parts of thewater-insoluble polymer solution obtained in Production Example 1 in53.9 parts of methyl ethyl ketone were 15.0 parts of a 5N sodiumhydroxide aqueous solution and 0.5 part of a 25% ammonia aqueoussolution both acting as a neutralizing agent as well as 341.3 parts ofion-exchanged water. Furthermore, 100 parts of C.I. Pigment Black 7(P.B. 7) as a carbon black pigment available from Cabot Corporation wasadded to the resulting mixture to prepare a pigment mixed solution. Thedegree of neutralization of the polymer in the thus prepared pigmentmixed solution was 78.8 mol %. The pigment mixed solution was mixed at20° C. for 1 hour using a disper blade operated at 7000 rpm. Theresulting dispersion was dispersed under a pressure of 180 MPa using aMicrofluidizer “High-Pressure Homogenizer M-140K” available fromMicrofluidics Corporation by passing the dispersion through the device15 times.

The thus obtained dispersion of the pigment-containing polymer particleswas held at 60° C. under reduced pressure to remove methyl ethyl ketonetherefrom, followed by further removing a part of water therefrom. Theresulting dispersion was subjected to centrifugal separation, and aliquid layer portion separated therefrom was subjected to filtrationtreatment through a filter “Minisart Syringe Filter” (pore size: 5 μm;material: cellulose acetate) available from Sartorius Inc., to removecoarse particles therefrom, thereby obtaining a water dispersion of thepigment-containing polymer particles. The solid content of the thusobtained water dispersion was 25% by mass. Then, 0.45 part of “DENACOLEX 321L” available from Nagase ChemteX Corporation and 15.23 parts ofion-exchanged water were added to 100 parts of the resulting waterdispersion of the pigment-containing polymer particles, and theresulting mixture was subjected to heat treatment at 70° C. for 3 hourswhile stirring. After cooling the obtained dispersion to roomtemperature, a liquid layer portion separated therefrom was subjected tofiltration treatment through a filter “Minisart Syringe Filter” (poresize: 5 μm; material: cellulose acetate) available from Sartorius Inc.,to remove coarse particles therefrom, thereby obtaining a waterdispersion of the pigment-containing polymer particles (solid content:22.00% by mass). The average particle size of the pigment-containingpolymer particles in the resulting water dispersion was 100 nm.

Production Examples 3 to 5 (Production of Water Dispersions of CyanPigment-, Magenta Pigment- and Yellow Pigment-Containing PolymerParticles)

The same procedure as in Production Example 2 was repeated except thatthe black pigment was replaced with a cyan pigment “PB. 15:3” availablefrom DIC Corporation, a magenta pigment “P.R. 150” available from FujiPigment Co., Ltd., and a yellow pigment “P.Y. 74” available fromDainichiseika Color & Chemicals Mfg. Co., Ltd., respectively, and theproduction conditions used therein were changed as shown in Table 1,thereby obtaining water dispersions of the pigment-containing polymerparticles (solid content: 22.00% by mass). The results are shown inTable 1.

TABLE 1 Production Production Production Production Example 2 Example 3Example 4 Example 5 Water Kind of pigment Black Cyan Magenta Yellowdispersion P.B. 7 P.B. 15:3 P.R. 150 P.Y. 74 Solid content (%) 22.0022.00 22.00 22.00 Ratio of pigment introduced (%) 68.76 68.52 68.5268.52 Composition Pigment (A) 15.13 15.08 15.08 15.08 (part(s) byPigment dispersing polymer 6.48 6.46 6.46 6.46 mass) (B-1)*¹ “DENACOL EX321L” 0.39 0.46 0.46 0.46 Ion-exchanged water 78.00 78.00 78.00 78.00Total 100.00 100.00 100.00 100.00 Properties Viscosity (mPa · s) 4.2 3.94.2 4 pH 9.9 9.8 9.9 9.9 Average particle size (nm) 100 100 155 115 Note*¹Water-insoluble polymer obtained in Production Example 1

Production Example 6 (Production of Fixing Aid Polymer Emulsion)

A 1000 mL separable flask was charged with 145 parts of methylmethacrylate available from Wako Pure Chemical Industries, Ltd., 50parts of 2-ethylhexyl acrylate available from Wako Pure ChemicalIndustries, Ltd., 5 parts of methacrylic acid available from Wako PureChemical Industries, Ltd., 18.5 parts of “LATEMUL E118B” (emulsifier;active ingredient content: 26%) available from Kao Corporation, 96 partsof ion-exchanged water and potassium persulfate available from Wako PureChemical Industries, Ltd., and the contents of the flask were stirredusing an agitation blade (300 rpm), thereby obtaining a monomeremulsion.

A reaction vessel was charged with 4.6 parts of “LATEMUL E118B”, 186parts of ion-exchanged water and 0.08 part of potassium persulfate, andan inside atmosphere of the reaction vessel was fully replaced withnitrogen gas. In a nitrogen atmosphere, the contents of the reactionvessel were heated to 80° C. while stirring with an agitation blade (200rpm), and then the aforementioned monomer emulsion was charged into adropping funnel and added dropwise into the reaction vessel over 3 hoursto subject the monomer emulsion to polymerization reaction. The obtainedreaction solution was mixed with ion-exchanged water, thereby obtaininga fixing aid polymer emulsion having a solid content of 41.6% by weight.The average particle size of the polymer particles in the resultingfixing aid polymer emulsion was 100 nm.

Production Example 7 (Production of Black Ink)

A mixed solution was prepared by mixing 4626.75 g of the waterdispersion of the black pigment-containing polymer particles (solidcontent: 22.0% by mass) obtained in Production Example 2, 438.87 g ofthe fixing aid polymer emulsion (solid content: 41.6% by weight)obtained in Production Example 6, 100.00 g of diethylene glycolmonoisobutyl ether, 2300.00 g of propylene glycol, 20.00 g of asilicone-based surfactant “KF-6011” available from Shin-Etsu ChemicalCo., Ltd., 100.00 g of an acetylene-based surfactant “SURFYNOL 440”available from Nissin Chemical Co., Ltd., and 2414.38 g of ion-exchangedwater with each other. The resulting mixed solution was subjected tofiltration treatment by passing through a filter “Minisart SyringeFilter” (pore size: 5.0 μm; material: cellulose acetate) available fromSartorius Inc., thereby obtaining a water-based ink. Various propertiesof the resulting water-based ink are shown in Table 2.

Production Examples 8 to 10 (Production of Cyan, Magenta and YellowInks)

The same procedure as in Production Example 7 was repeated except thatthe water dispersion of the black pigment-containing polymer particleswas replaced with the respective water dispersions containing the cyanpigment-, magenta pigment- and yellow pigment-containing polymerparticles, and the production conditions used therein were changed asshown in Table 2, thereby obtaining cyan, magenta and yellow water-basedinks. The results are shown in Table 2. In addition, the four color inksobtained in Production Examples 7 to 10 were used as an ink set 1.Various properties of the ink set 1 are shown in Table 2.

Production Examples 11 to 14, 15 to 18 and 19 to 22

The same procedures as in Production Examples 7 to 10 were repeatedexcept that the four color inks obtained in Production Examples 11 to14, the four color inks obtained in Production Examples 15 to 18 and thefour color inks obtained in Production Examples 19 to 22, which wereproduced under the conditions shown in Table 3, were used as an ink set2, an ink set 3 and an ink set 4, respectively. The results are shown inTable 2.

TABLE 2 Ink set 1 Ink set 2 Production Example No. 7 8 9 10 11 12 13 14Kind of ink Black Cyan Magenta Yellow Black Cyan Magenta Yellow InkBlack pigment-containing water 10.18 — — — 10.18 — — — compositiondispersion (part(s) by Cyan pigment-containing water — 10.22 — — — 10.22— — mass) dispersion Magenta pigment-containing — — 10.22 — — — 10.22 —water dispersion Yellow pigment-containing — — — 10.22 — — — 10.22 waterdispersion Fixing aid polymer emulsion 1.82 1.78 1.78 1.78 1.82 1.781.78 1.78 Propylene glycol 23.00 22.00 21.00 20.00 23.00 23.00 23.0023.00 Diethylene glycol monoisobutyl 1.00 2.00 3.00 4.00 1.00 1.00 1.001.00 ether Silicone-based surfactant 0.20 0.30 0.40 0.50 0.20 0.20 0.200.20 Acetylene-based surfactant 1.00 1.00 1.00 1.00 0.80 1.00 1.20 1.40Ion-exchanged water 62.80 62.70 62.60 62.50 63.00 62.80 62.60 62.40Properties Average particle size (nm) 96.2 101.3 155.6 116.7 95.9 100.6156.5 113.2 Viscosity at 32° C. (mPa · s) 4.8 4.4 4.9 5.1 4.6 4.2 4.64.7 Static surface tension (mN/m) 28.0 27.5 26.7 25.9 28.9 28.1 27.627.2 pH 9.6 9.4 9.6 9.3 9.6 9.4 9.7 9.4 Ink set 3 Ink set 4 ProductionExample No. 15 16 17 18 19 20 21 22 Kind of ink Black Cyan MagentaYellow Black Cyan Magenta Yellow Ink Black pigment-containing water10.18 — — — 10.18 — — — composition dispersion (part(s) by Cyanpigment-containing water — 10.22 — — — 10.22 — — mass) dispersionMagenta pigment-containing — — 10.22 — — — 10.22 — water dispersionYellow pigment-containing — — — 10.22 — — — 10.22 water dispersionFixing aid polymer emulsion 1.82 1.78 1.78 1.78 1.82 1.78 1.78 1.78Propylene glycol 23.00 23.00 23.00 23.00 23.00 23.00 23.00 23.00Diethylene glycol monoisobutyl 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00ether Silicone-based surfactant 0.20 0.20 0.20 0.20 0.00 0.00 0.00 0.00Acetylene-based surfactant 1.00 1.00 1.00 1.00 0.80 1.00 1.20 1.40Ion-exchanged water 62.80 62.80 62.80 62.80 63.20 63.00 62.80 62.60Properties Average particle size (nm) 96.6 98.6 150.5 117.4 93.2 101.6148.8 112.5 Viscosity at 32° C. (mPa · s) 4.7 4.1 4.5 4.5 4.5 4.2 4.64.7 Static surface tension (mN/m) 28.0 28.0 27.9 28.0 30.9 30.2 29.328.6 pH 9.6 9.4 9.7 9.4 9.5 9.3 9.5 9.2

Examples 1 and 2 and Comparative Example 1

Using the ink set 1 obtained in Production Examples, characters orimages were printed on a corona-treated PET “TAIKO Polyester FilmFE2001” (water absorption of the printing medium as measured bycontacting the printing medium with pure water for 100 milliseconds: 0g/m²) available from Futamura Chemical Co, Ltd., by the followingink-jet printing method.

(Ink-Jet Printing Method)

Under the environmental conditions of a temperature of 25±1° C. and arelative humidity of 30±5%, the water-based inks were loaded into aone-pass system print evaluation apparatus available from Trytech Co.,Ltd., equipped with ink-jet print heads “KJ4B-HDO6MHG-STDV”(piezoelectric type) available from Kyocera Corporation. These ink-jetprint heads were sequentially disposed in the print evaluation apparatusin the order of the black ink, cyan ink, magenta ink and yellow ink froman upstream side in the feed direction of the printing medium. At thistime, the ink-jet print heads respectively loaded with the black ink,cyan ink, magenta ink and yellow ink were arranged in the printevaluation apparatus such that adjacent two of the ink-jet print headswere spaced at a distance of 55 cm apart from each other.

The operating conditions of the print evaluation apparatus were set to ahead applied voltage of 26 V, a frequency of 10 kHz, an ejected inkdroplet amount of 3 pL, a head temperature of 32° C., a resolution of600 dpi, a number of ink shots for flashing before being ejected of 200shots and a negative pressure of −4.0 kPa, and the printing medium wasfixed in the print evaluation apparatus such that the longitudinaldirection of the printing medium was aligned with a transportationdirection thereof.

The print evaluation apparatus was equipped with the ink-jet print headsfor ejecting the water-based inks onto the printing medium and an underheater for heating the printing medium from a rear side surface of theprinting medium opposed to a front side surface thereof which faces tothe respective ink-jet print heads. The distance between the underheater and the printing medium was set to 0.25 mm, and the distancebetween the respective ink-jet print heads and the printing medium wasset to 1.0 mm. In addition, the surface temperature of the under heaterwas set to 20° C., 40° C. or 60° C.

An A4-size film heater available from Kawai Corporation was fixedlymounted to a transportation table for the printing medium so as to heatthe printing medium. The temperature of the film heater was set to 20°C., 40° C. or 60° C., and the transportation speed of the printingmedium was adjusted to 25 m/min.

Then, a printing command was transmitted to the print evaluationapparatus to eject the black ink, cyan ink, magenta ink and yellow inktherefrom in this order. As a result, solid images of the respectivecolor inks were printed on the printing medium such that the solidimages of the different two color inks among the four color inks wereformed in a perpendicularly overlapped relation to each other to therebyprepare three print patterns as shown in FIG. 1. Thereafter, the thusprepared print patterns were dried by a hot air dryer at 60° C. for 5minutes, thereby obtaining a printed material.

The resulting printed material was subjected to evaluation of colorbleeding, evaluation of deformation of the substrate by heating andevaluation of uniformity of solid images according to the followingmethods and evaluation ratings. The results are shown in Table 3.

(Method of Evaluating Color Bleeding of Printed Material)

In a portion of the printed material where the two solid images of thedifferent color inks were perpendicularly overlapped each other tothereby cause color bleeding, the width of a protruded portion of one ofthe color inks from the perpendicularly overlapped portion of the twosolid images in which the solid image of one of the color inks wasintruded into the solid image of the other of the color inks wasmeasured. The width of the protruded portion was measured with respectto each combination of the different two color inks among the four colorinks to calculate an average value thereof. The degrees of colorbleeding caused in the printed material were compared based on the thusobtained average value, and evaluated according to the followingevaluation ratings.

A: The average value of the widths of the protruded portions was lessthan 80 μm, and substantially no color bleeding was recognized.

B: The average value of the widths of the protruded portions was notless than 80 μm and less than 100 μm, and no remarkable color bleedingwas recognized, so that the resulting printed material was usable inpractical applications without problems.

C: The average value of the widths of the protruded portions was notless than 100 μm and less than 200 μm, and although slight colorbleeding was recognized, the resulting printed material was still usablein practical applications without problems.

D: The average value of the widths of the protruded portions was notless than 200 μm, and color bleeding clearly occurred so that theresulting printed material was unusable in practical applications.

(Evaluation Ratings for Deformation of Printed Material by Heating)

A: No deformation of the printed material was observed.

B: Deformation of the printed material was observed.

(Evaluation Ratings for Uniformity of Solid Image Printed)

A: No mottling nor uneven density was observed in a solid image printedportion.

B: Mottling or uneven density was observed in less than 10% of an areaof the solid image printed portion, but the resulting printed materialwas still usable in practical applications without problems.

C: Mottling or uneven density was observed in not less than 10% of anarea of the solid image printed portion, and significant problems werecaused upon using the printed material in practical applications.

Examples 3 and 4 and Comparative Example 2

The same procedure in Example 2 was repeated except that the ink set wasreplaced with the respective ink sets shown in Table 3.

Example 5 and Comparative Example 3

The same procedure in Example 2 was repeated except that thecorona-treated PET as the printing medium was replaced with a coronadischarge-treated OPP “TAIKO Polypropylene Film FOR-AQ” available fromFutamura Chemical Co, Ltd., and further in Comparative Example 3, thetemperature of the under heater was changed to 80° C.

TABLE 3 Com. Com. Com. Ex. 1 Ex. 2 Ex. 1 Ex. 3 Ex. 2 Ex. 4 Ex. 5 Ex. 3No. of ink set used  1  1  1  2  3  4  1  1 Temperature of under heater(° C.) 40 60 20 60 60 60 60 80 Printing medium used PET PET PET PET PETPET OPP OPP Evaluation Color bleeding B A D B D C A B resultsDeformation of printed A A A A A A A B material Uniformity of solidimage A A A A A B A A printed

As is apparent from the comparison of Examples 1 and 2 with ComparativeExample 1, the comparison of Examples 3 and 4 with Comparative Example 2and the comparison of Example 5 with Comparative Example 3 as shown inTable 3, it was confirmed that the ink-jet printing methods of theExamples hardly suffered from color bleeding or deformation of theprinted materials and were capable of obtaining good printed charactersor images which were excellent in uniformity, as compared to the ink-jetprinting methods of the Comparative Examples.

INDUSTRIAL APPLICABILITY

According to the ink-jet printing method of the present invention, it ispossible to obtain good printed characters or images which are free ofnot only thermal deformation of a low-liquid printing medium, but alsomottling or color bleeding.

The invention claimed is:
 1. An ink-jet printing method comprising:providing an ink-jet printing apparatus equipped with an ink-jet printhead that is capable of ejecting water-based inks onto a low-liquidabsorbing printing medium; providing water-based inks each comprising acolorant (A), an organic solvent (C), and water, wherein a staticsurface tension of the respective water-based inks as measured at 20° C.is in the range 22 mN/m through 45 mN/m and wherein a difference instatic surface tension between each of said three or more kinds ofwater-based inks is not less than 0.1 mN/m; providing a low-liquidabsorbing printing medium; loading the ink-jet printing apparatus withthree or more kinds of said water-based inks which are different instatic surface tension from each other; heating the printing medium to atemperature of from 30 to 75° C.; and ejecting the water-based inks inthe sequential order from the water-based ink having a higher staticsurface tension, thereby printing characters or images on the low-liquidabsorbing printing medium.
 2. The ink-jet printing method according toclaim 1, wherein the colorant (A) in the respective water-based inks ispresent in the form of pigment-containing water-insoluble polymerparticles.
 3. The ink-jet printing method according to claim 1, whereinthe organic solvent (C) comprises a glycol ether (c-2).
 4. The ink-jetprinting method according to claim 1, wherein a boiling point of theorganic solvent (C) is not lower than 90° C. and lower than 250° C. 5.The ink-jet printing method according to claim 1, wherein a content ofthe organic solvent (C) in the respective water-based inks is not lessthan 15% by mass and not more than 45% by mass.
 6. The ink-jet printingmethod according to claim 1, wherein the water-based inks each furthercomprise a polymer (B).
 7. The ink-jet printing method according toclaim 1, wherein the water-based inks each further comprise a surfactant(D).
 8. The ink-jet printing method according to claim 7, wherein thesurfactant (D) comprises a silicone-based surfactant (d-1).
 9. Theink-jet printing method according to claim 1, wherein a static surfacetension of the respective water-based inks as measured at 20° C. is notless than 25 mN/m and not more than 35 mN/m.
 10. The ink-jet printingmethod according to claim 1, wherein a difference in static surfacetension between the three or more kinds of water-based inks is not lessthan 03 mN/m.
 11. The ink-jet printing method according to claim 1,wherein a viscosity of the respective water-based inks as measured at32° C. is not less than 2.0 mPa·s and not more than 12 mPa·s.
 12. Theink-jet printing method according to claim 1, wherein the low-liquidabsorbing printing medium is a polyester film subjected to coronadischarge treatment or an oriented polypropylene film subjected tocorona discharge treatment.
 13. The ink-jet printing method according toclaim 1, wherein the printing medium is heated by the ink-jet print headthat is capable of ejecting the water-based inks onto the printingmedium and an under heater that is disposed on a rear side surface ofthe printing medium opposed to a front side surface thereof which facesto the ink-jet print head.
 14. The ink-jet printing method according toclaim 1, wherein a printing speed of printing the characters or imageson the printing medium is not less than 5 m/min and not more than 75m/min in terms of a transportation speed of the printing medium.
 15. Theink-jet printing method according to claim 1, wherein the characters orimages are printed on the printing medium by a single-pass (one-pass)method.
 16. The ink-jet printing method according to claim 1, whereinthe respective colorant-containing inks are ejected onto the printingmedium that is not subjected to any pretreatments.
 17. The ink-jetprinting method according to claim 1, wherein a content of the pigment(A) in the respective water-based inks is not less than 2.0% by mass andnot more than 30.0% by mass.
 18. The ink-jet printing method accordingto claim 1, wherein a content of the polymer (B) in the respectivewater-based inks is not less than 1.0% by mass and not more than 20% bymass.
 19. The ink-jet printing method according to claim 1, wherein acontent of the surfactant (D) in the respective water-based inks is notless than 0.1% by mass and not more than 5% by mass.
 20. The ink-jetprinting method according to claim 1, wherein a content of water in therespective water-based inks is not less than 20% by mass and not morethan 85% by mass.
 21. The ink-jet printing method according to claim 1,wherein said low-liquid absorbing printing medium is a non-liquidabsorbing medium.